Stereochemistry and Stereoelectronics of Azines. 13. Conformational Effects on the Quadrupolarity of Azines. An Ab Initio Quantum-Mechanical Study of a Lateral Synthon

Abstract

The quadrupole moment of formaldazine, H2C=N-N=CH2, has been studied for the trans structure (Ð(C-N-N-C) = τ = 180≡) and a series of gauche structures (τ > 120≡). Restricted Hartree-Fock theory, second-order Moller-Plesset theory, and quadratic CI theory have been used in conjunction with the basis sets 6-31G*, 6-31G**, 6-311G** and 6-311++G**. Formaldazine is a quadrupolar molecule with primitive quadrupole moment tensor components of Qxx = -22.4, Qyy = -20.4 and Qzz = -25.6 DA at the theoretical level QCISD/6-311++G**. The examination of the theoretical level dependency shows that the reliable computation of a quadrupole moment requires the use of a flexible basis set. A large part of the component Qzz = -25.6 DA is due to the π-system and compares, on a per electron basis, with the Qzz value of benzene. Conformational changes of the azines in the range 120° < τ < 180≡ have but a minute effect on the energy and are associated with only minor electronic relaxation. These conformational changes alter the quadrupole moment tensor components less than ΔQxx = +0.4, ΔQyy = +1.6 and ΔQzz = -1.0 DA at QCISD/6-311++G**//QCISD/6-31G*. The direction of these changes is explained by consideration of the rotation of the CN-π-systems and a small reduction of the CN bond polarity in the gauche structures. The Qzz component of formaldazine is representative of the quadrupole moment tensor component along the direction of the C2 axis of the azine bridge as such. Hence, the results of this study suggest that azines can engage in strong quadrupole-quadrupole interactions and can be employed as lateral synthons in crystal engineering.